Japanese has a rich set of focus particles, several exclusive and additive particles, and, in addition, contrastive particles. This thesis provides a formal description of the meanings of Japanese focus particles and addresses two general questions: "What kinds concepts do Japanese focus particles express?" and "Why does Japanese have a larger inventory of focus particles than English?" Focus particles are typically classified into exclusive particles like "only," non-scalar additive particles like "also," and scalar additive particles like "even." In addition to such particles, Japanese form class of focus particles includes contrastive particles. One of the most general semantic properties of focus particles is to relate the new proposition containing the focused constituent with contextually available propositions containing alternatives. For exclusive particles, propositions containing alternatives are excluded from the set of true propositions. For additive particles, propositions containing alternatives are part of the set of true propositions. Scalar additive particles further impose an ordering between the newly introduced proposition and propositions in the context based on the relative likelihood or informativeness of propositions. As an answer to the first question, "What kinds concepts do Japanese focus particles express?", this thesis shows that in addition to the kinds of concepts expressed by exclusive, non-scalar and scalar additive particles, Japanese contrastive particles express the notion of relevance. For contrastive particles, the speaker does not comment on whether or not propositions containing alternatives are part of the set of true propositions. Instead, what contrastive particles express is the relative relevance of the newly introduced proposition and propositions in the context. As an answer to the second question, "Why does Japanese have a larger inventory of focus particles than English?", this thesis shows that, first, the meanings of some Japanese focus particles are more restricted than those of their English counterparts. For example, for one Japanese scalar additive particle, "-made," the contextually available proposition containing an alternative has to be presupposed while for "even," it can be either inferred from the newly introduced proposition or presupposed. The second reason for the larger inventory of Japanese focus particles is that aside from the common semantic function of focus particles which is to relate the new proposition and structurally related propositions in the context, the meanings of some Japanese focus particles involve a third contextual proposition. For example, contexts in which one Japanese exclusive particle, "-shika," is acceptable, are characterized by the presence of a proposition which does not hold in the context. The meanings of contrastive particles also involve a third contextual proposition because to evaluate the relative relevance between two propositions, not only two propositions but also a third proposition with respect to which the relative relevance of the two propositions is evaluated, are necessary. By investigating in detail the semantic properties of each of Japanese exclusive, scalar additive, and contrastive particles, this study expands our understanding of what focus particles can express. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]